ninja/mac: Allow per configuration SDKROOT.

This CL allows some flags to be per configuration instead of per target, so
iOS ninja can build simulator and device.

Make generator keeps its old behavior. Make doesn't know the build configuration
at gyp time and supporting this at build time is difficult.

R=thakischromium.org

Review URL: https://codereview.chromium.org/24080003



git-svn-id: http://gyp.googlecode.com/svn/trunk@1737 78cadc50-ecff-11dd-a971-7dbc132099af